Portable phones, which typically run off three NiCd cells, ideally require linear regulators that will operate down to a 3-V output with a super low dropout voltage (as low as possible). To answer that call, this circuit (see the figure) works for output voltages down to 3.0 V at load currents up to 130 mA.
The battery in today's phones provides a nominal voltage of about 3.6 V, but will drop to about 3.2 V at the end of discharge. In addition, most portable phones are designed using internal regulated rails in the 3-V to 3.3-V range, but require the phone to operate (possibly at reduced performance) as the rail drops down to about 3 V. The low battery voltage sets the dropout voltage requirement at about 150 m V maximum (for load currents that are typically in the 100-to-150-mA range).
Although tested at 3.0 V, the circuit shown will work equally well (with the same low dropout voltage characteristics) at higher output voltages. The dropout voltage is load-current dependent, and doesn't depend on the output voltage.
Taking a closer look, the basic regulator used is the LP2951, which is an adjustable-output-voltage, 100-mA regulator. To get the very low dropout voltage, an external pnp transistor is used at Q1 (test data was taken for three different types of transistors, see the table).
The VBE needed to turn on Q1 is developed across R2, which means that regulator U2's input must sink slightly more than a milliampere to get enough voltage across R to turn on Q1. As the load current increases, U1 sinks more current as required to drive the base of Q1. This current flows from the output pin of U1 and through R3 to ground.
R4 and R5 sense the output voltage and provide the 1.23-V feedback voltage to U1 that's needed for regulation. The values shown will set the output voltage to 3 V (within tolerance of the resistors and the LP2951 reference). For the testing of this circuit, R4 was trimmed as required to set the output precisely. Capacitors C1 and C2 are necessary for regulator stability, and R1 holds the shutdown pin low (keeping the LP2951 on). To get the exact values for dropout voltage, the regulated output was trimmed to 3.031 V with the input at 5 V. The input voltage was then carefully reduced until the output read 3.000 V (which meant the output was "dropping out" of regulation), and the input voltage was recorded.
The voltage difference between input and output was defined as the dropout voltage:
Vdrop = Vin - Vout (@ Vout = 3.000 V and IL = 130 mA).
As shown in the table, three different transistor types at Q1 were tested. Test conditions were as follows:
TA = 25°C RL = 23.2 Ω; IL = 130 mA; and VO = 3.000 V.